C.sub.8 alkylaromatic hydrocarbons are generally considered to be valuable products, and para-xylene in particular is in high demand. On the other hand, C.sub.9 and C.sub.10 alkylaromatic hydrocarbons are not nearly as valuable but are typically produced as a byproduct in the same aromatic production processes used to produce C.sub.8 alkylaromatic hydrocarbons. Various approaches have been used to convert the less valuable C.sub.9 and C.sub.10 alkylaromatic hydrocarbons into C.sub.8 alkylaromatic hydrocarbons. One popular approach has been to transalkylate C.sub.9 and C.sub.10 alkylaromatic hydrocarbons along with benzene or toluene to form the C.sub.8 alkylaromatic hydrocarbons. Specifically, trimethylbenzenes and tetramethylbenzenes have been transalkylated along with benzene and toluene to form xylenes. However, transalkylation reactions are equilibrium limited and the product contains a mixture of unreacted C.sub.9 and C.sub.10 alkylaromatic hydrocarbons along with the desired C.sub.8 alkylaromatic hydrocarbons. To increase conversion, commercial processes have utilized a two-stage design with the first stage being a fixed bed reactor and the second stage being a separation unit. Unreacted C.sub.9 and C.sub.10 alkylaromatic hydrocarbons present in the reactor product stream are separated and recycled to the reactor; see for example U.S. Pat. No. 3,211,798.
The present invention makes use of reversible flow reactive chromatography to perform the transalkylation. Reactive chromatography in general allows for concurrent reaction and separation of the unconsumed reactants from products, thereby extending product yields beyond thermodynamic equilibrium limitations. Reactive chromatography has been applied to other classes of chemical reactions; see for example U.S. Pat. No. 3,122,494 which describes an isomerization process having two sub-beds containing a mixture of catalyst and adsorbent where the feed is introduced between the two sub-beds and the desorbent introduction is alternated between the first sub-bed and the second sub-bed. The adsorbent must selectively adsorb straight-chain hydrocarbons to the substantial exclusion of non-straight-chain hydrocarbons.
The present invention expands the application of reversible flow reactive chromatography to an entirely new class of chemical reactions, the transalkylation of alkylaromatic hydrocarbons. U.S. Pat. No. 2,836,633 describes alkylation of aromatic hydrocarbons using a catalyst supported on an adsorbent, but the purpose of using the adsorbent support was to increase the activity of the catalyst, not to perform an actual separation. CA2031096 and AU9067656-A describe an alkylation process where an alkene is adsorbed on a dry cationic exchange resin in its hydrogen form and is simultaneously reacted with an aromatic hydrocarbon. The present invention uses reversible flow and the combination of catalyst and adsorbent in a single zone both to effect transalkylation and to concurrently separate the unconsumed C.sub.9 and C.sub.10 alkylaromatic hydrocarbons from the C.sub.8 alkylaromatic hydrocarbon products.